File Name: anatomy and physiology of the endocrine system .zip
The endocrine system consists of glands widely separated from each other with no physical connections Fig. Homeostasis of the internal environment is maintained partly by the autonomic nervous system and partly by the endocrine system.
The autonomic nervous system is concerned with rapid changes, while endocrine control is mainly involved in slower and more precise adjustments. Although the hypothalamus is classified as a part of the brain rather than an endocrine gland, it controls the pituitary gland and has an indirect effect on many others. The ovaries and the testes secrete hormones associated with the reproductive system after puberty.
The endocrine glands are explored in the early sections of the chapter. Problems that arise when abnormalities occur are usually caused by the over- or under-activity of endocrine glands and are explained in the final sections of the chapter.
The receptors for peptide hormones are situated on the cell membrane and those for lipid-based hormones are inside the cell. The level of a hormone in the blood is variable and self-regulating within its normal range.
This may be controlled either indirectly through the release of hormones by the hypothalamus and the anterior pituitary gland, e. The pituitary gland and the hypothalamus act as a unit, regulating the activity of most of the other endocrine glands.
It is the size of a pea, weighs about mg and consists of three distinct parts that originate from different types of cells. There is a network of nerve fibres between the hypothalamus and the posterior pituitary. Figure 9. This is supplied by branches from the internal carotid artery. The anterior lobe is supplied indirectly by blood that has already passed through a capillary bed in the hypothalamus but the posterior lobe is supplied directly.
This comes from both lobes, containing hormones, and leaves the gland in short veins that enter the venous sinuses between the layers of dura mater.
The influence of the hypothalamus on the release of hormones is different in the anterior and posterior lobes of the pituitary gland. This is supplied indirectly with arterial blood that has already passed through a capillary bed in the hypothalamus Fig. These hormones influence secretion and release of other hormones formed in the anterior pituitary. The lobes of the pituitary gland and their relationship with the hypothalamus. Synthesis and storage of antidiuretic hormone and oxytocin. Table 9.
Posterior pituitary hormones are synthesised in the nerve cell bodies, transported along the axons and stored in vesicles within the axon terminals in the posterior pituitary Fig. Their release by exocytosis is triggered by nerve impulses from the hypothalamus. Some of the hormones secreted by the anterior lobe stimulate or inhibit secretion by other endocrine glands target glands while others have a direct effect on target tissues. This in turn stimulates the target gland to produce and release its hormone.
As a result the blood level of that hormone rises and inhibits the secretion of releasing factor by the hypothalamus Fig. This is the most abundant hormone synthesised by the anterior pituitary. It stimulates growth and division of most body cells but especially those in the bones and skeletal muscles. Body growth in response to the secretion of GH is evident during childhood and adolescence, and thereafter secretion of GH maintains the mass of bones and skeletal muscles.
It also regulates aspects of metabolism in many organs, e. Secretion of GH is greater at night during sleep and is also stimulated by hypoglycaemia, exercise and anxiety. The daily amount secreted peaks in adolescence and then declines with age. Inhibition of GH secretion occurs by a negative feedback mechanism when the blood level rises and also when GHRIH is released by the hypothalamus.
This hormone is synthesised by the anterior pituitary and its release is stimulated by thyrotrophin releasing hormone TRH from the hypothalamus. Release is lowest in the early evening and highest during the night. Secretion is regulated by a negative feedback mechanism Fig. When the blood level of thyroid hormones is high, secretion of TSH is reduced, and vice versa. ACTH levels are highest at about 8 a. This circadian rhythm is maintained throughout life. It is associated with the sleep pattern and adjustment to changes takes several days, following, e.
Secretion is also regulated by a negative feedback mechanism, being suppressed when the blood level of ACTH rises Fig. Other factors that stimulate secretion include hypoglycaemia, exercise and other stressors, e. Immediately after birth, suckling stimulates prolactin secretion and lactation. The resultant high blood level is a factor in reducing the incidence of conception during lactation.
Prolactin, together with oestrogens, corticosteroids, insulin and thyroxine, is involved in initiating and maintaining lactation. Prolactin secretion is related to sleep, i. At puberty secretion increases, further enabling normal adult functioning of the reproductive organs; in both males and females the hormones responsible are:. Stimulates production of sperm in the testes, stimulates secretion of oestrogen by the ovaries, maturation of ovarian follicles, ovulation.
Stimulates secretion of testosterone by the testes, stimulates secretion of progesterone by the corpus luteum. These hormones act directly on non-endocrine tissue and their release from synaptic vesicles by exocytosis is stimulated by nerve impulses from the hypothalamus. Oxytocin stimulates two target tissues during and after childbirth parturition : uterine smooth muscle and the muscle cells of the lactating breast.
Sensory impulses are generated and travel to the control centre in the hypothalamus, stimulating the posterior pituitary to release more oxytocin. The process of milk ejection also involves a positive feedback mechanism. Suckling generates sensory impulses that are transmitted from the breast to the hypothalamus.
The impulses trigger the release of oxytocin from the posterior pituitary and oxytocin stimulates contraction of the myoepithelial cells around the glandular cells and ducts of the lactating breast to contract, ejecting milk. The role of this hormone in males and non-lactating females remains unclear. The main effect of antidiuretic hormone is to reduce urine output diuresis is the production of a large volume of urine.
ADH acts on the distal convoluted and collecting ducts of the nephrons of the kidneys Ch. As a result, the reabsorption of water from the glomerular filtrate is increased.
As the osmotic pressure rises, for example as a result of dehydration or following haemorrhage, the secretion of ADH increases. More water is therefore reabsorbed and the urine output is reduced. This means that the body retains more water and the rise in osmotic pressure is reversed. Conversely, when the osmotic pressure of the blood is low, for example after a large fluid intake, secretion of ADH is reduced, less water is reabsorbed and more urine is produced Fig.
At high concentrations, for example after severe blood loss, ADH causes smooth muscle contraction, especially vasoconstriction in small arteries. Anterior view.
The thyroid gland is situated in the neck in front of the larynx and trachea at the level of the 5th, 6th and 7th cervical and 1st thoracic vertebrae. It is a highly vascular gland that weighs about 25 g and is surrounded by a fibrous capsule.
The superior thyroid artery is a branch of the external carotid artery and the inferior thyroid artery is a branch of the subclavian artery. Two parathyroid glands lie against the posterior surface of each lobe and are sometimes embedded in thyroid tissue. The gland is composed of cuboidal epithelium that forms spherical follicles.
Iodine is essential for the formation of the thyroid hormones, thyroxine T 4 and tri-iodothyronine T 3 , so numbered as these molecules contain four and three atoms of the element iodine respectively. The thyroid gland selectively takes up iodine from the blood, a process called iodine trapping. Otherwise, it remains fairly constant throughout life. Thyroid hormones enter the cell nucleus and regulate gene expression, i.
They enhance the effects of other hormones, e. This hormone is secreted by the parafollicular or C-cells in the thyroid gland Fig. It promotes storage of calcium in bones and inhibits reabsorption of calcium by the renal tubules. Its effect is opposite to that of parathyroid hormone, the hormone secreted by the parathyroid glands.
Release of calcitonin is stimulated by an increase in the blood calcium levels. This hormone is important during childhood when bones undergo considerable changes in size and shape.
There are four small parathyroid glands, two embedded in the posterior surface of each lobe of the thyroid gland Fig. They are surrounded by fine connective tissue capsules. The cells forming the glands are spherical in shape and are arranged in columns with sinusoids containing blood between them.
Secretion is regulated by blood calcium levels. When they fall, secretion of PTH is increased and vice versa. The main function of PTH is to increase the blood calcium level when it is low. This is achieved by indirectly increasing the amount of calcium absorbed from the small intestine and reabsorbed from the renal tubules.
If these sources provide inadequate supplies then PTH stimulates osteoclasts bone-destroying cells and calcium is released from bones into the blood. Parathormone and calcitonin from the thyroid gland act in a complementary manner to maintain blood calcium levels within the normal range. This is needed for:. The two adrenal suprarenal glands are situated on the upper pole of each kidney Fig. They are about 4 cm long and 3 cm thick.
The right gland drains into the inferior vena cava and the left into the left renal vein. The glands are composed of two parts which have different structures and functions. The adrenal cortex is essential to life but the medulla is not. The adrenal cortex produces three groups of steroid hormones from cholesterol. They are:.
The endocrine system consists of glands widely separated from each other with no physical connections Fig. Homeostasis of the internal environment is maintained partly by the autonomic nervous system and partly by the endocrine system. The autonomic nervous system is concerned with rapid changes, while endocrine control is mainly involved in slower and more precise adjustments. Although the hypothalamus is classified as a part of the brain rather than an endocrine gland, it controls the pituitary gland and has an indirect effect on many others. The ovaries and the testes secrete hormones associated with the reproductive system after puberty. The endocrine glands are explored in the early sections of the chapter.
In order to survive, animals must constantly adapt to changes in the environment. The nervous and endocrine systems both work together to bring about this adaptation. In general the nervous system responds rapidly to short-term changes by sending electrical impulses along nerves and the endocrine system brings about longer-term adaptations by sending out chemical messengers called hormones into the blood stream. In general Endocrine system is represented by a set of heterogeneous structure and origin of formations capable of internal secretion, ie the release of biologically active substances hormones that flow directly into the bloodstream. For example, think about what happens when a male and female cat meet under your bedroom window at night. The initial response of both cats may include spitting, fighting and spine tingling yowling - all brought about by the nervous system.
Communication is a process in which a sender transmits signals to one or more receivers to control and coordinate actions. Together, these two systems are primarily responsible for maintaining homeostasis in the body. The nervous system uses two types of intercellular communication—electrical and chemical signaling—either by the direct action of an electrical potential, or in the latter case, through the action of chemical neurotransmitters such as serotonin or norepinephrine. Neurotransmitters act locally and rapidly. When an electrical signal in the form of an action potential arrives at the synaptic terminal, they diffuse across the synaptic cleft the gap between a sending neuron and a receiving neuron or muscle cell. Once the neurotransmitters interact bind with receptors on the receiving post-synaptic cell, the receptor stimulation is transduced into a response such as continued electrical signaling or modification of cellular response.
Human Physiology pp Cite as. The biological role of the endocrine system is closely linked to that of the nervous system; the two together coordinate the functions of the other in some cases widely separated organs and organ systems. The distinguishing feature of the endocrine system is that its influence is exerted by way of a number of substances, the hormones. Chemically, the hormones are a nonuniform group; the range of compounds represented includes steroids, amino-acid derivatives, peptides and proteins. They have specific actions on these target organs, actions that as a rule cannot be produced by any other substance.
Ageing negatively affects hormone secretion by the glands of the endocrine system, which makes older people more prone to insomnia, fractures, diabetes and cognitive changes. Like all other body systems, the endocrine system undergoes age-related changes that negatively affect its functioning. As a result of these changes, older people are more prone to disturbed sleep patterns, have a reduced metabolic rate, lose bone density, accumulate body fat, and show increases in blood glucose. As a consequence, they are at higher risk of health issues such as insomnia, fractures, type 2 diabetes and cognitive decline.
Human endocrine system , group of ductless glands that regulate body processes by secreting chemical substances called hormones. Hormones act on nearby tissues or are carried in the bloodstream to act on specific target organs and distant tissues. Diseases of the endocrine system can result from the oversecretion or undersecretion of hormones or from the inability of target organs or tissues to respond to hormones effectively. It is important to distinguish between an endocrine gland, which discharges hormones into the bloodstream, and an exocrine gland , which secretes substances through a duct opening in a gland onto an external or internal body surface.
If your institution subscribes to this resource, and you don't have a MyAccess Profile, please contact your library's reference desk for information on how to gain access to this resource from off-campus. Please consult the latest official manual style if you have any questions regarding the format accuracy. A hormone is a chemical that is produced by the body and has a specific regulatory effect on a target cell or organ. Classic endocrinology was concerned with the functions of anatomically defined glands such as the thyroid gland or the pituitary gland. It is now recognized that almost every organ secretes hormones and that endocrine cells may be dispersed throughout the body e. The more recent study of endocrinology encompasses all processes concerned with the physiology of hormones.
Endocrine glands are ductless glands of the endocrine system that secrete their products, hormones , directly into the blood. The major glands of the endocrine system include the pineal gland , pituitary gland , pancreas , ovaries , testes , thyroid gland , parathyroid gland , hypothalamus and adrenal glands. The hypothalamus and pituitary glands are neuroendocrine organs.
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Издать? - Он с сомнением покачал головой. - Издать. - Некоторые идеи о протоколах вариативных фильтров и квадратичных остатках. - Стопроцентный бестселлер. Она засмеялась.
Похож на китайца. Японец, подумал Беккер. - Бедняга. Сердечный приступ. Беккер безучастно кивнул: - Так мне сказали. Лейтенант вздохнул и сочувственно помотал головой.
Однако тот не подавал никаких признаков жизни. Сьюзан перевела взгляд на помост перед кабинетом Стратмора и ведущую к нему лестницу. - Коммандер.